I've had a leaver in 3 of my last 8 ranked matches. Twice on my team, once on the enemy team. While this is more frequent than usual, experiencing a leaver is not uncommon. I would estimate ~5% of my matches have a leaver in them. That is incredibly high, and I experience Leavers in League far more than other games I play. Clearly, not enough is being done to address this issue. Not only does it ruin a match, it almost guarantees a loss for your team, sapping -17 or so ELO from four innocent players.
I'm not sure what the % us before someone gets put into low priority queue, but a low priority queue is a joke punishment. And it takes several leaves before you even get there. Players don't care about abandoning matches, because the system is so forgiving there's no punishment.
**Here is my suggestion**
_For the player who leaves a match_ (in addition to current low priority queue):
1) First offense, 200% ELO loss
2) Second (or greater) offense within a 4 month period, 300% ELO loss
_For the leaver's teammates*:_
ELO loss reduced by 50%
*Anyone in a party with the leaver experiences 100% ELO loss, to avoid abuse.